This invention relates to an illumination optical system and, more particularly, to an illumination optical system having a laser as a light source thereof and suitable to be applied to an exposure apparatus for the manufacture of integrated circuit devices.
Recent trends of further miniaturization of a pattern on an integrated circuit has forced development of optical lithography using a light beam in the deep UV region, i.e. having a wavelength in the range of 200-330 nm. In such optical lithography, a super Hg lamp or an Xe-Hg lamp is often used as the light source. Since, however, the Hg lamp or Xe-Hg lamp has substantially no directionality and provides poor luminance, use of such a lamp as the light source results in a prolonged exposure time and, thus, in a decreased throughput.
Development of lasers providing laser beams in shorter wavelength regions has been advanced, so that it is now possible to introduce a laser of high luminance in the deep UV region, as described, into an exposure apparatus for the manufacture of semiconductor circuit devices.
In the field of exposure apparatuses, it is known to use an optical system to form a plurality of secondary light sources in order to prevent image distortion on the wafer surface owing to the diffraction effect caused by the pattern of the mask.
More specifically, in a high resolution imaging optical system such as employed in the exposure apparatus, the ratio between the numerical aperture of the illumination optical system and the numerical aperture of the projection optical system has a significant affect on the imaging performance. If the numerical aperture of the illumination optical system is small as compared with that of the projection optical system, there occurs a higher degree of distortion in the pattern projected, though the contrast is improved. For this reason, the ratio is usually set in the range of approx. 0.3-0.8. In order to achieve such a ratio, the light emitting portion of the light source in the illumination system must have some degree of expansion. In view of this, a plurality of point light sources are formed with minute gaps maintained therebetween to define a light source having an apparent expansion. The light beams emitted from such an apparently expanded light source are superposed, by means of optics, upon one another on the surface to be irradiated.
Such a technique of forming plural secondary light sources would not be directly applied to an exposure apparatus employing a laser as the light source means. This is because, if the laser beam emitted from a single laser source is divided to define a plurality of secondary light sources, the split laser beams emerging from the secondary light sources would interfere with each other due to high coherence of the laser beam so that interference fringes would appear on the mask surface, resulting in enormously uneven exposure.